The invention relates in general to a cutting tap, and in particular to a cutting tap having a cutting edge geometry that improves the resistance of the cutting edges to chipping and fracture.
Mechanisms and machine components requiring screw threads have a long history in technology. Specifically, the application of screw threads as fastener components dominates over all other means to join parts into assemblies. Although there are many ways to generate screw threads both internal as well as external, experience has shown that taps are the favored means to generate the internal screw thread. There currently exist two tapping methods to generate internal screw threads. The dominant tapping method is by cutting and removing material from the walls of a hole to produce a helical V-shaped screw thread. Alternatively, internal screw threads can be created by displacing material to form an internal screw thread. However, tapping by cutting material is generally favored because this method requires lower torque and produces a more perfect thread form.
The dimensional accuracy of the shape and size of the internal screw thread controls the precision and fit of the screw thread assembly. Additionally, the speed of tapping affects the cost to produce an internal screw thread.
There are two materials used to manufacture cutting taps. High-speed steel is widely used for taps because of its high strength. However, cemented tungsten carbide is favored as a material for manufacturing other cutting tools over high-speed steel owing to properties such as higher hardness and high temperature stability including the ability to retain hardness at high temperatures. Typically, cutting tools manufactured from cemented carbide can be used at cutting speeds that are at least three times higher than tools manufactured from “high-speed” steel and the life of the tool is longer.
Referring now to FIGS. 9-11, there is shown one flute of a four-fluted prior art cutting tap that has a straight cutting face. In general, the cutting tap generates an internal thread form by a succession of cutting edges on the chamfered section of the tap having a length L. Material is removed from the wall of the hole until the final thread form is obtained with the first full thread on the main body of the tap. This progressive formation of an internal thread is illustrated in FIG. 9 by superimposing the sections of material removed by each of the four flutes.
As shown in FIG. 10, the prior art cutting tap has a straight cutting face that is inclined relative to a radial reference line that travels from the cutting edge at the major diameter to the center of the cutting tap at a cutting angle (or rake angle) A1. In FIG. 10, the cutting angle A1 is defined as the included angle between a line passing along the surface of the cutting face and the radial reference line. The cutting angle A1 is positive when the inclination from the radial reference line is in the counterclockwise direction as viewed in FIG. 10. The cutting angle A1 is negative when the inclination from the radial reference line is in the clockwise direction as viewed in FIG. 10.
The magnitude of the cutting angle A1 has an influence on edge strength of the prior art cutting tap. In this regard, one can increase the strength of the cutting edge by reducing the cutting angle A1 (i.e., making the cutting angle A1 more negative). However, while a reduction in the cutting angle A1 will increase the strength of the cutting edge, the amount of cutting force necessary to tap (or cut) the threads increases with the reduction in the cutting angle A1. When taps of the prior art are manufactured from cemented carbide, the cutting edges are very prone to chipping because carbide has low strength as compared to high-speed steel. Specifically, the cutting edges that are most prone to chipping are the narrow edges on the chamfer that approach and include the first full thread after the chamfer. The narrow full threads after the chamfer are also prone to chipping because they have a small included angle. The wider edges on the entry part of the chamfer are far less prone to chipping because they are not as narrow as the cutting edges of the full threads.
It should be appreciated that the above description of the obstacles connected with the cutting angle A1 of a cutting tap that has a straight cutting face also exist for a cutting tap that has an arcuate cutting face. In this regards, for a cutting tap that has an arcuate cutting face, a chordal hook angle corresponds to the rake angle A1 for the cutting tap with the straight cutting face. The chordal hook angle is defined as the angle between a radial reference line between the major diameter to the center of the cutting tap and a chord between the distal cutting edge and the minor diameter of the cutting tap.
As shown in FIG. 11, the cutting edges of the conventional cutting tap are prone to chipping, especially the narrow cutting edges on the chamfer that approach and include the first full thread after the chamfer (illustrated by the third chamfered thread in FIG. 11). The wider cutting edges on the entry part of the chamfer are less prone to chipping (illustrated by the first and second chamfered thread). Prior art taps have a chamfer defined by a single straight line at a chamfer angle A2 with respect to the axis of the tap. Because the chamfer is straight, the thickness T1 of the sections of material removed by each chamfered cutting edge remains constant.
Because taps are geometrically weak, especially the cutting edges, they are prone to chipping. Because cemented carbide has lower strength than high-speed steel, taps made from cemented carbide are more prone to chipping than taps made from high-speed steel. Therefore, it is not possible to currently use taps made from cemented carbide in some applications where high-speed steel taps can be used.